Water-absorbing resins have found wide use in sanitary goods, hygenic goods, water retaining agents, dehydrating agents, sludge coagulants, thickening agents, condensation preventing agents and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms including substituted and unsubstituted natural and synthetic polymers such as hydrolysis products of starch-acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidines and polyacrylonitriles.
Each type of water-absorbing resin differs in ease and cost of manufacture, chemical and physical properties, rate of water-absorption, and degree of water-absorption and retention. For example, the hydrolysis products of starch-acrylonitrile graft polymers have a comparatively high ability to absorb water, but require a cumbersome process for production and have the disadvantages of low heat resistance and decaying or decomposing easily due to the presence of starch. Conversely, other water-absorbent polymers are easily and cheaply manufactured and are not subject to decomposition, but do not absorb liquids as well as the starch-acrylonitrile graft polymers.
Therefore, it would be extremely advantageous to provide a method of increasing the water absorption properties of a stable, easy to manufacture waterabsorbing resin to match the superior water-absorption properties of a difficult to manufacture polymer. Likewise, it would be advantageous to increase the liquid absorption properties of an already superior water-absorbent resin.
One of the processes for polymerizing acrylic acid and acrylates is aqueous solution polymerization. The polymer obtained by this process is soluble in water and, therefore, is crosslinked to modify the polymer into a useful water-absorbing resin. However, even if the modification is effected by reacting a crosslinking agent concurrently with or after aqueous solution polymerization, the resulting reaction product is in the form of a difficult to handle, highly viscous aqueous solution, or a gel, containing absorbed water. As a result, the aqueous solution or gel must be dehydrated (dried) to obtain a water-absorbing resin in the desired solid or powder form. However, it is difficult to dry the reaction product efficiently by the usual rotary drum roller method or spray drying method because care must be taken to avoid the excessive crosslinking that results from over-heating during drying. Furthermore, insufficient drying of the resin results in reduced crosslinking density. Therefore, extreme difficulties are encountered in preparing a resin having a desired low water content and good water-absorbing ability.
Any method of improving the water-absorbing properties of a resin must also retain the "dry feel" of the resin after liquid absorption. Although water and liquid absorption is the primary function of the water-absorbing resin, in many applications it is almost equally important that the polymer maintain its "dry feel". The polymer must be able to absorb amounts of water several times its weight, plus be sufficiently crosslinked to avoid partial solubilization of the polymer to form a gel and lead to a slippery, wet feeling. Presently, water-absorbing resins, such as crosslinked polyacrylic acid, do possess a "dry feel" after significant water absorption. Thus any methods directed to improving the water-absorbing properties of such a resin should not alter the basic "dry feel" of the resin after liquid absorption.
Any method that both increases the water-absorbing capabilities of a water-absorbent resin and maintains the basic "dry feel" of the resin would enhance and broaden the application possibilities of many water-absorbent polymers. Such a method should be simple and economical to avoid increases in the raw material or the manufacturing cost of the water-absorbent resin.